In many engineering disciplines, one or more housing or case elements are mechanically conjoined to form a chamber wherein the functional components of a device are housed. It is often the case that one or more of the housing elements is itself a functional component of a device. One such example is the housing elements of a computer head disc assembly (HDA). Typically, in most current production HDAs, at least one of the case elements also serves as a mounting frame for mounting at least some of the HDA functional components thereto.
It is often desirable that the housing elements, when mechanically conjoined, provide not only a stable mechanical, electrical, hydraulic, pneumatic or other platform in which the device's functional components operate, but that the housing elements unite to form a sealed environment for maintaining those functional components in some form of controlled environment. This environment may require the retention of a fluid or liquid within the sealed housing, the exclusion of fluids, liquids, or contaminants from without the sealed housing, or a combination of both requirements.
Many HDAs in current production are sealed within two horizontally divided case halves. In a typical instance, a first case element has disposed upon it hard drive functional elements including, but not limited to, at least one magnetically recordable and readable hard disk, a motor to drive the disk, a read/write head mounted on an arm which may be in turn pivotably mounted on the case element, as well as other sundry electro-mechanical and electronic components. A second case element, which may or may not have additional functional elements disposed thereon, is then positioned over the first case element and mechanically secured in place with a plurality of mechanical fasteners, e.g. machine screws.
The plurality of machine screws provide a vertical clamping pressure between the first and second horizontally separated case elements. Captured between the first and second case elements and forming a seal therebetween is often a conformable, or deformable, gasket element. Examples of currently utilized gaskets include but are not limited to die-cut sheet gaskets, cast resilient polymeric gaskets and O-rings. The first and second case elements in operative combination with the screws applying a vertical force to the gasket element seal the cavity formed within the first and second case elements. This has the effect of precluding the unwanted intrusion of contaminants from the ambient atmosphere into the case interior.
Additionally, the case elements/screws/gasket element in operative combination often unite to provide an effective electromagnetic interference (EMI) barrier. EMI can be a significant problem in computer head disc assemblies in that excess levels of EMI can serve to apply an unwanted signal or noise to interior components of the HDA. Such noise can interfere with the proper read/write functionality of the HDA.
The case sealing methodology previously outlined presents several problems during the manufacture of HDAs. First, in order to ensure an effective seal between the first and second case elements, a significant number of screws is often required to obtain the required screw clamping pressure about the periphery of the case elements as well as to obviate case deformation occasioned by that pressure.
A second problem is that during the manufacturing process of HDAs, it is occasionally necessary to rework an HDA being built. This rework is often effected subsequent to a final quality assurance check conducted subsequent to the final assembly of the drive, which check reveals some malfunction. Accordingly, in order to rework a defective drive component after final assembly, it is generally necessary to remove the plurality of screws uniting the first and second case elements in operative combination and to separate those case elements. As the number of screws required to mechanically connect the case elements increases, so increases both the amount of time required to assemble and disassemble the drive, as well as the likelihood that the assembly or disassembly process will damage the screw threads of either the screw itself or of the case elements. Furthermore, it is often the case that a deformable gasket captured and clamped between a first and second case element is permanently deformed thereby or is damaged by the separation of the case elements, necessitating the replacement of the gasket during rework. Each of these factors adds to the time, expense and effort required to rework an HDA.
One alternative to the use of vertically clamped gasket elements in HDA manufacturing technology has been the use of a metalized pressure sensitive adhesive tape applied to the seam between case elements subsequent to their being mechanically conjoined by a plurality of screws. The use of such tape can reduce the number of screws required to mechanically conjoin and seal the several case elements, and the metallic surface provides the requisite EMI barrier. By removing the requirement for an even clamping pressure across a broad horizontal surface, the number of screws required to join the case elements is reduced. Because the pressure sensitive adhesive tape is metalized, it further acts as an effective block to EMI.
While the previously discussed use of pressure sensitive adhesive tape provides an advantage over vertically clamped gaskets, this solution presents a new set of problems to HDA designers. A first problem is occasioned by the fact that the removal of the tape and/or the adhesive residue therefrom during rework constitutes a further increase in labor costs during rework. Further, pieces of tape removed during rework may inadvertently be introduced into the case interior, requiring further effort. A third new problem is occasioned by the fact that adhesives in general, and pressure sensitive adhesives in particular, exhibit an out-gassing of volatile adhesive components, especially the adhesive's solvent or carrier. These volatile components, when admixed with the ambient atmosphere inside the HDA and dispersed therethrough can adversely effect critical electronic or magnetic components contained inside the HDA.
What is needed is some means to quickly and securely seal and unseal HDA case elements. This sealing methodology should reduce the number of screws or other mechanical fasteners required to conjoin the case elements. The methodology should further minimize damage to the case elements in the event it becomes necessary to separate them during rework or repair. The seal should leave no appreciable residue behind on being removed, and should be easily sealed and unsealed. The seal should not introduce any appreciable levels of volatile solvents into the interior of the HDA. It would be additionally advantageous if the seal were reusable: failing that, its removal should not introduce any additional components into the HDA. Finally, the sealing methodology should reduce electromagnetic interference beneath an acceptable threshold level, or preclude it entirely. It would be further desirable if a sealing methodology could facilitate alignment of the several seal elements during construction, and optimally minimize unwanted opening of, or tampering with, the seal.